1. Field of the Invention
The teachings herein relate to a hand held detector of ionizing radiation and more particularly to a detector for discriminating a gamma component and a neutron component.
2. Description of the Related Art
Detection of radioactive materials, particularly those illicitly hidden in the stream of commerce, requires the availability of a variety of radiation detection equipment. In particular, Hand-Held RadioIsotope Identification Devices (HHRIID) are needed in the field to quickly determine the presence of special nuclear material and distinguish it from the presence of medical and industrial radioisotopes, as well as from normally occurring radioactive material. One possible embodiment of an HHRIID consists of two optically separated radiation sensors that emit light and are coupled to a common photodetector. The first radiation sensor is a neutron sensing component that contains atomic nuclei with a high neutron cross section, such as 6Li in a chemical compound, such as 6LiF, surrounded by particles of a scintillator material, for example, ZnS:Ag, and bound together in an epoxy matrix. The second radiation sensor is a gamma sensing component and consists of a scintillator crystal with enhanced gamma energy resolution, high gamma stopping power, and an atomic composition with very low neutron absorption cross section. The two radiation sensors are optically separated in such a manner that the light emitted by one sensor does not reach the other sensor in order to avoid optical crosstalk. The HHRIID may include a pulse shape discrimination circuit that identifies the source of light emitted (either by the neutron sensing component or the gamma sensing component based on the difference in scintillation light decay times.)
One issue associated with HHRIID applications that has not been previously addressed is the radiation cross talk in the neutron sensing component. Even though by design the neutron sensing component is typically sub-millimeter thin and contains atoms with low-Z numbers, it is still sensitive to gamma rays. In many field applications, it is possible that the incident gamma flux is high enough to create a significant number of interactions in the neutron sensing component, thereby impeding the detection and measurement of low neutron fluxes that may be present at the same time.
What is needed is a compact, integrated HHRIID design that minimizes or eliminates the radiation cross talk in the neutron sensing component, thus enabling improved analyses of the various components of a mixed radiation field.